1. Field of the Invention
The present invention relates to an optical apparatus, such as a video camera, a silver-halide camera, an electronic still camera or a lens barrel adapted for such apparatuses.
2. Description of Related Art
In the field of the optical apparatuses such as cameras and the like, a trend of reducing the size of a photo-taking optical system and the diameter of the image size of a solid-state image sensor has rapidly advanced during recent years. Meanwhile, plastic materials are often employed as optical materials for forming photo-taking optical systems.
The use of a plastic material as an optical material facilitates molding lenses with dies in desired shapes and also at a lower cost than the use of a glass material. On account of these advantages, lenses made of plastic materials are often used for forming a viewfinder system, an infrared active automatic focusing unit and also some part of a photo-taking optical system.
However, the variations of physical properties of plastic materials due to variations of environment are greater than those of inorganic glass materials. In respect of a coefficient of linear expansion, for example, a plastic material called "PMMA" is 67.9.times.10.sup.-6 /.degree. C. in representative value, while that of an inorganic glass material called "LaK 14" (manufactured by OHARA Co.) is 57.times.10.sup.-7 /.degree. C., which is smaller than the former by one place. Further, with respect to variations of refractive index due to variations of temperature, the value of the material "PMMA" is 1.0 to 1.2.times.10.sup.-4 /.degree. C., while that of the inorganic glass material "LaK 14", in D line, is 3.9 to 4.4.times.10.sup.-6 /.degree. C., which is smaller than the former by two places.
The optical constants (such as a refractive index and shape, etc.) of the plastic material are thus caused to vary by temperature variations to a much greater extent than those of the inorganic glass material. Therefore, compared with a lens made of an inorganic glass material, the so-called plastic lens which is made of a plastic material varies in focal length, for example, to a greater extent for variations of temperature.
Further, the water absorption of the plastic material is greater than that of the inorganic glass material. Therefore, variations of humidity, like temperature, also causes the optical constants of a plastic lens to vary to a greater extent than those of a lens made of an inorganic glass material.
Hence, an optical system consisting in part of a plastic lens is inferior in optical performance to an optical system consisting of lenses made of an inorganic glass material, as the optical properties such as focal length, etc., of the former will be caused to vary by temperature and humidity variations to a greater degree than those of the latter.
Further, the same variations of optical constants as those mentioned above tend to result from use of a plastic material also for a lens holding member arranged to hold a lens.
Meanwhile, some of optical apparatuses, such as a video camera or the like, have presented a problem in that the lens control of the apparatus cannot be adequately accomplished as the image forming position (focus position) of an optical system is caused to deviate by variations in environmental conditions. The adverse effect of such focus deviation due to variations of environment becomes more conspicuous, because the quality of images has come to be improved further by the advancement of digital technology. This problem, therefore, has become serious.
Manufacturers are these days competing for increase in zoom magnifying rate and decrease in size of an optical system. As a result, however, the amount of defocus on a prescribed image forming plane due to variations of temperature or humidity has increased to present a very serious problem.
In a case where a focus driving range of a focusing lens is set for a normal temperature, in particular, if an image forming plane is caused to shift by variations of environment (variations in ambient temperature or humidity), it becomes hardly possible to adequately carry cut a focusing action.
To solve the above-stated problem, the focus driving range may be set to include some allowance for the shift of the image forming plane due to variations of environment. Adoption of such a solution, however, would cause the focusing lens to be driven to such a position that is located outside of a range of focusing positions and would require an unnecessarily long time in focusing on an object image. That solution lowers the performance of an AF function and is, therefore, not desirable for an optical apparatus.
In another conceivable solution, the focusing lens driving range may be set and kept in storage in such a way as to include all in-focus positions that correspond to such variations in in-focus position that are expected to be caused by variations of temperature. However, the arrangement of storing all such in-focus positions that correspond to variations of temperature requires a huge storage capacity, which would result in an excessive increase in cost and price of the optical apparatus.